In the recent years, there has been a lot of development in the area of compact color illumination devices. As a result, highly collimated compact color illumination devices have found application in all areas where the conventional illumination devices have failed to meet user requirements. These areas include mobile display devices such as a micro-projectors and the flashlight for camera embedded in cell phones. Other application areas include biosensors, and instruments used in medical imaging and surgery. Conventional illumination devices have high power consumption, provide low optical output and low lighting efficiency, and have a bulky size. Therefore, conventional illumination devices are not suitable for use in pocket-sized or other small instruments and equipments.
A full color micro-projector utilizes color sequential technology to obtain improved optical power efficiency and image quality. The color sequential technology uses time division multiplexing to produce colors. A repetitive sequence of red, green, and blue sub-frames is created sequentially in the micro-projector such that the human eye integrates the sub-frames into full-color frames. With the help of the color sequential technology, high display resolution and color saturation can be obtained.
In the current development of compact illumination devices, a package of Red-Green-Blue (RGB) color light emitting chips, arranged in the shape of a triangle, is utilized to achieve color mixing. The triangle arrangement of the package results in each of the light-emitting chips competing for the center spots. As a result, the emitted light patterns do not align perfectly because of the spatial separation of the RGB chips. The light output efficiency also suffers because of the triangle topology.
Furthermore, most light sources for illumination are Lambertian sources, and the intensity of the Lambertian source is directly proportional to the cosine of the angle between the viewing direction and the surface normal to the light source. Lambertian illumination sources cannot be used efficiently in applications such as micro-projectors because the light cone would have to be modified by complicated optics due to the wide angle distribution of the light emission. Moreover, it is difficult to collimate such a color illumination source with high accuracy.
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